The quality of residue feeds, particularly bitumen (heavy oil), suffers from high levels of heteroatoms (sulfur, nitrogen and oxygen) and metals (nickel, vanadium and iron). Refining and/or conversion of such sulfur-laden crudes is costly due to the hydrogen needed to remove the sulfur. As environmental pressures continue to lower allowable emission levels in mogas and diesel products, refining costs continue to rise.
Penalty costs for sulfur-laden feeds in refineries can be exorbitant. Hence, deep desulfurization of such feeds has become a critical research target. Thus, there is a need for low cost processes which upgrade oils to more environmentally friendly and more profitable feedstocks.
Much work has been done utilizing molten caustic to desulfurize heavy oils. For example, see "Molten Hydroxide Coal Desulfurization Using Model Systems," Utz, Friedman and Soboczenski, 51-17 (Fossil Fuels, Derivatives, and Related Products, ACS Symp. Series., 319 (Fossil Fuels Util.), 51-62, 1986 CA 105(24):211446Z); "An Overview of the Chemistry of the Molten-caustic Leaching Process," Gala, Hemant, Srivastava, Rhee, Kee, Hucko, and Richard, 51-6 (Fossil Fuels, Derivatives and Related Products), Coal Prep. (Gordon & Breach), 71-1-2, 1-28, 1989 CA112(2):9527r; and "Base-catalyzed Desulfurization and Heteroatom Elimination from Coal-model Heteroaromatic Compounds," 51-17 (Fossil Fuels, Derivatives, and Related Products, Coal Sci. Technol., 11 (Int. Conf. Coal Sci., 1987), 435-8, CA108(18):153295y).
Additionally, work has been done utilizing aqueous caustic to desulfurize shale and coal. U.S. Pat. No. 4,437,980 discusses desulfurizing, deasphalting and demetallating shale and coal in the presence of molten potassium hydroxide, hydrogen and water at temperatures of about 350.degree. C. to about 550.degree. C. U.S. Pat. No. 4,566,965 discloses a method for removal of nitrogen and sulfur from oil shale with a basic solution comprised of one or more hydroxides of the alkali metals and alkaline earth metals at temperatures ranging from about 50 to about 350.degree. C.
Methods also exist for the regeneration of aqueous alkali metal, see e.g., U.S. Pat. No. 4,163,043 discussing regeneration of aqueous solutions of Na, K and/or ammonium sulfide by contact with Cu oxide powder yielding precipitated sulfide which is separated and re-oxidized to copper oxide at elevated temperatures and an aqueous solution enriched in NaOH, KOH or NH.sub.2. Romanian patent RO-101296-A describes residual sodium sulfide removal wherein the sulfides are recovered by washing first with mineral acids (e.g., hydrochloric or sulfuric acid) and then with sodium hydroxide or carbonate to form sodium sulfide followed by a final purification comprising using iron turnings to give insoluble ferrous sulfide.
Sodium metal desulfurization is also disclosed in U.S. Pat. Nos. 3,785,965, 3,787,315, 3,788,978, 3,791,966, 3,796,559, 4,076,613 and 4,003,824.
U.S. Pat. No. 4,003,823 discloses a process for desulfurizing and hydroconverting heavy feeds by contacting the feed at elevated temperature with alkali metal hydroxides in the molten state. Water is tolerated as an impurity but only up to 15 wt % water based on alkali metal hydroxide, and has a suppressing effect when present in greater than 20%. The (Col. 8, 1. 64-68 etc.) patent teaches the presence of liquid, molten or vapor phases, but expressly teaches away from the operability of a substantially aqueous NaOH.
What is needed is a continuous process for removal of organically bound sulfur which further allows for recovery and regeneration of the desulfurizing agents, and which reduces the amount of sodium metal needed for use in the desulfurizing processes. Processes that reduce the need for sodium metal treatments in the desulfurization process are highly desirable.